15 Dec
2008
15 Dec
'08
5:27 p.m.
On Dec 15, 2008, at 5:21 PM, Tony Duell wrote:
[NLS's products]
[pardon me for jumping in, but I know a little about the
instrument in question]
It uses relays to switch resistors in and out of what amounts to a
discrete implementation of a Kelvin bridge, with the relay
configuration reflected in the displays in much the same way as you
describe the mechanical bridge in the paragraph below. I believe
(but am not sure) that the null detector was a simple comparator
circuit.
The displays are the engraved plate style, edge-lit using
incandescent lamps.
I've been looking for schematics for this instrument for many
years but have not been able to find them.
And before that, very early digital voltmeters...
How did they work? I rmember seeing (and alas failing to obtain) an
partically mechanical digital voltmeter. The circuitry was a servo
system, similar to the ones used in chart recorders. The motor drove a
mechanicla turns counter, coupled to an accurate multi-turn helical
potentiometer, which provided the feedback to the servo amplifier (it
replaced the slidewire in th chart recorder implementation). THe
operation is obvious, apply an input voltage, the motor turns until
the
feedback from the pot balances the input votlage, the turns counter
then
effectively shows where the pot is
Nifty!
-Dave
--
Dave McGuire
Port Charlotte, FL